Carbon nanotube reinforced polymer and process for preparing the same

ABSTRACT

Carbon nanotube reinforced polymers include a polymer and carbon nanotubes reinforcing the polymer. The carbon nanotube reinforced polymer exhibits a conductivity percolation threshold of less than 10 6  Ω/cm at a carbon nanotube content of 1.5 wt. % and less. The polymer may be selected from a polyamide or a polystyrene based polymer. In certain embodiments, the carbon nanotube content is between 0.1 to 1.5 wt. %, and the reinforced polymer will have a percolation threshold at a carbon nanotube content of less than 0.5 wt. %.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of commonly owned U.S. application Ser.No. 10/545,315, filed Apr. 10, 2006, now U.S. Pat. No. 7,569,637, whichin turn is the national phase application under 35 USC §371 ofPCT/NL2004/000109 filed Feb. 12, 2004 which designated the U.S. andclaims benefit of PCT/NL03/00108, dated Feb. 13, 2003 andPCT/NL03/00584, dated Aug. 15, 2003, the entire contents of eachapplication being hereby incorporated by reference herein.

FIELD

The present invention relates to a process for the preparation of acarbon nanotube reinforced polymer.

BACKGROUND AND SUMMARY

In recent years, much effort has been put into the incorporation ofcarbon nanotubes in polymer matrices. The composites obtained areinteresting materials, since they have enhanced electrical andmechanical properties at very low loading due to the specific nanotubecharacteristics, such as their high aspect ratio and electricalconductance. However, dispersion of carbon nanoutubes in highly viscouspolymers is difficult and has often been attempted by functionalizingthe nanotubes, leading to attractive interactions between the nanotubesand the polymer. In addition, dispersing exfoliated single nanotubes hasbeen found to be a challenge, since nanotubes are highly bundled as aresult of strong van-der-Waals interactions.

In general, materials can be divided into three groups regarding theirelectrical conductivity δ: insulators (δ<10⁻⁷ S/m), semi-conductors(δ=10⁻⁷−10⁵ S/m) and conductors (δ>10⁵ S/m). For polymers, typicalconductivity values range from 10⁻¹⁵ S/m up to 10⁻¹² S/m. Carbon fillerscan have conductivities in the range of 10⁴ S/m up to 10⁷ S/m. Incomposites, the conductivity levels off to a slightly lower value thanfor the pure carbon species at higher filler concentration.

Carbon nanotube reinforced polymers are presently made by incorporatingcarbon nanotubes (CNTs), generally in the form of a bundle, in a polymermatrix. In order to obtain a homogeneous distribution of these CNTs,they are pretreated by either an ultrasonic treatment, or by a chemicalmodification process, aimed at improving the dispersability of theindividual CNT in the polymer matrix. The incorporation of CNTs in sucha polymeric matrix is for the enhancement of the stiffness as well asthe conductivity of the polymer matrix material.

The reported procedures for obtaining homogeneous dispersions of CNTs inpolymer matrices result in either breaking and lowering of the aspectratio of the tubes (which is unfavourable for stiffness, strength, andconductivity of the composite), or in damaging the surface of the tubes(which lowers the stability and the conductivity of the tubes).

The process of the present invention offers a solution to this problem,as a result of which the CNTs remain substantially of the same lengthand aspect ratio. The reinforced polymer resulting from the process ofthe present invention has enhanced conductive and mechanical properties.

In J. Mater. Sci, 37, 2002, pages 3915-23, a process is described forthe preparation of a poly(styrene/butyl acrylate) copolymer nanocomposite using CNTs as filler. This process uses multiwall CNTs (MWNT),suspended in an aqueous solution of sodium dodecyl sulphate (SDS), and alatex of the copolymer. An amount of at least 3 wt. % of the MWNT isneeded to have a significant change in the electrical conductivity ofthe nanocomposite (the so-called percolation threshold).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the resistivity measurements made in accordancewith Example II/comparative experiment A below; and

FIG. 2 is a graph of the resistivity measurements made in accordancewith Examples III-V below.

DETAILED DESCRIPTION

The process of the present invention provides a carbon nanotubesreinforced polymer having a percolation threshold at significantly lowerloading of the CNT. It also provides a carbon nanotubes reinforcedpolymer based on other suspensions of the CNT, as well as other latexesor precursors thereof.

The process of the present invention comprises the following steps:

-   A) contacting carbon nanotubes in an aqueous medium with a    water-soluble component, comprising either a water-soluble first    polymer, or a water soluble surfactant;-   B) mixing the resulting product from step A) with either an aqueous    latex of a second polymer, or with (a) water-soluble precursor(s) of    a second polymer;-   C) removing water from the so obtained mixture;-   D) heating the product from step C) to a temperature at which the    second polymer flows or where the second polymer is formed from out    of its precursor(s); and-   E) processing and/or solidifying the product from step D) into a    desired form. The steps of the process of the present invention will    be separately discussed below.

Step A): preparing a slurry from carbon nanotubes in an aqueous medium.This method is described in WO 02/076888. In this publication a methodis described for the exfoliation of single wall carbon nanotubes (SWNT),resulting in a stable aqueous product containing essentially singletubes. In this publication a water-soluble polymeric material is usedfor obtaining the exfoliated nanotubes, The contents of this publicationare incorporated herein by reference.

In the process of the present invention the use of SWNTs is preferred,as it results in a much lower amount of the CNTs needed for obtainingthe percolation threshold of the CNT-reinforced polymer, compared to theuse of MWNTs. This lower loading also improves the mechanical and flowproperties of the reinforced polymers.

As described in WO 02/076888, the water-soluble polymeric materialshould preferably be of a hydrophilic nature, either from natural orsynthetic origin. In the process according to the present invention itis advantageous that the first polymer is Gum Arabicum.

In the present invention it has shown to be advantageous to improve theincorporation of the water-soluble polymeric material, when itshydrophilic nature is to be improved, to add (e.g. in step A) anelectrolyte, like a water-soluble salt, like sodium-chloride. Thisimproves the dispersability of the CNT in the matrix of the carbonnanotubes reinforced polymer.

In step A also a water-soluble surfactant can be used to effectivelyexfoliate the CNTs. Preference is given to a salt of a hydrocarbonsulphate or sulphonate, like sodiumdodecyl sulphate (SDS) orsodiumdodecyl sulphonate. Also preferred is a polyalkyleneoxide basedsurfactant.

The process of the present step A) is performed by contacting theessential ingredients (the water-soluble polymer or surfactant, and thecarbon nanotubes) in any order in water or an aqueous solution. Theresulting product can obtain up to 75 weight % of carbon nanotubes,coated with the said first polymer or surfactant. In this process, stepA) the mass ratio of the first polymer or surfactant to the carbonnanotubes can range from 0.05 to 20.

The temperature at which this step A) is performed is not critical.Temperatures between room temperature and 75° C. are very well suited.

The residence time needed for an effective exfoliation of the carbonnanotubes can be easily determined by a man skilled in the art.Residence times below 1 hour have proven to be sufficient for thatpurpose.

Step B): The product resulting from step A) is brought into contact witheither an aqueous latex of a second polymer, or with (a) water-solubleprecursor(s) of a second polymer. This second polymer is the polymerwhich constitutes the matrix of the carbon nanotubes reinforced polymer,in which the carbon nanotubes are well-dispersed. Every aqueous polymerlatex known to the skilled man can be used. Preference is given to asecond polymer being selected from the group comprising polyacrylates,styrene-based (co-)polymers, butadiene-based (co-)polymers,polycarbonate, acrylonitrile-based (co-)polymers, (halogen-containing)polyolefins (like polyethylene or polypropylene), and polyamides.

Also (a) precursor(s) for such a second polymer can be used, as theyare, or in the form of an aqueous solution thereof which can beconverted to the second polymer via an emulsion polymerization.Preference can be given, for instance when a nylon is used as the secondpolymer, to the use in this step B) of either the monomer of saidpolymer (like ε-caprolactam when using nylon 6 as the final matrixmaterial), or to the use of a salt of adipic acid and hexamethylenediamine, or diaminobutane, when nylon 6,6 or nylon 4,6 as the matrixmaterial is aimed at. The skilled man is aware of the precursor(s)needed for such a second polymer. A preference is given to the use inthis step B) of ((a) precursor(s) of) a polyamide or a polystyrene basedpolymer.

The temperature of this step B) generally lies between 10 and 150° C.,The pressure is generally atmospheric, but may be increased in order toaccommodate for processability in this step B) or in the following stepC). The residence time for this step B) is not critical, and generallydoes not exceed 1 hour.

Although both thermoset polymers as well as thermoplastic polymers canbe used as the matrix of the CNT reinforced polymer, the preference isgiven to the use of a (semi-) crystalline or amorphous thermoplasticpolymer.

Step C): the mixture obtained in process step B), according to thepresent invention, is treated to remove (substantially all of the)water. There are different physical methods available to the skilled manto achieve this removal. Out of these methods, a preference is forperforming step C) by means of evaporation, freeze-drying, orflash-drying.

Step D): is intended to realize a homogeneous dispersion of the CNTs inthe second polymer. When in the preceeding steps use is made of (a)precursor(s) for this second polymer, this step D) is also intended toform the second polymer from this/these precursor(s). In the case thatthe second polymer is a thermoplastic polymer, the temperature in thisstep D is chosen such that it is 10-100° C. above the melting point (incase of a (semi-)crystalline second polymer), or above the glass point(in case of an amorphous second polymer). In the case that the secondpolymer is a thermoset polymer, the temperature in this step D) ischosen such, that this second polymer can be formed from itsprecursor(s), during which formation also step E) of the process of thepresent invention is applied.

In all cases, the man skilled in the art is aware of the processconditions under which this step D) is to be performed, depending on thenature of the second polymer.

Step E): of the process of the present invention is the processingand/or solidification of the product of step D) in a desired form. Thisstep E) can be a molding step, a pelletizing step, an injection orcompression molding step, or any known step to form a solidified polymerobject.

The process of the present invention results in a CNT reinforcedpolymer, wherein the properties of the CNTs used are retained: the CNTsare hardly or not damaged, as a result of which they retain theiroriginal length as well as their original aspect ratio (AR) (ratio oflength to diameter of the CNTs). The CNTs are essentially individuallydispersed in the polymer matrix. The polymer therefore has improvedstiffness as well as better conductivity properties.

The invention also relates to a carbon nanotube reinforced polymer,obtainable by the process of the present invention. With the (process ofthe) present invention polymer composites are obtainable having aconductivity percolation threshold at or below 3 wt. % of CNT. Inparticular, the process of the present invention results in a productthat has a resistivity of less than 10⁶ Ω/cm at a carbon nanotubecontent of less than 3 wt. %, preferably 1.5 wt. % and less, morepreferably between 0.1 to 1.5wt. %. In the art such a resistivity isonly achieved at much higher loadings of the CNT, as can be seen fromthe article in J. Mater. Sci (supra).

The present invention therefore also relates to a carbon nanotubesreinforced polymer having a Relative Size Dimension (RSD) of thenanotubes incorporated therein of between 0.85 and 1.0, wherein the RSDis the ratio between the AR of the nanotubes in the reinforced polymer,and the AR of the virgin nanotubes (the CNTs used as starting materialin the process of the present invention). More pronounced, the CNTreinforced polymer of the present invention has an RSD of at least 0.9.

The reinforced polymer of the present invention can be used for severalapplications in which the improved stiffness and conductivity propertiescan be exploited, Reference can be given to shielding applications (likeelectromagnetic interference shielding); high modulus conducting bodypanels for the automotive industry with a better surface appearance thanglass fibre filled polymers; nano-electric devices (such as thin-filmtransitors), and others.

The invention is illustrated by the following non-limiting Examples andcomparative experiment.

EXAMPLE I

Materials and Techniques

Materials: CNT−AP grade (Carbolex) (a SWNT), and Gum Arabicum, (GA)(Aldrich) were used as received.

An aqueous product of CNT+GA was prepared according to the teachings ofWO 02/076888. GA was dissolved in water at room temperature to formsolutions of 0.5 wt % to 15 wt %. A powder of as-produced single wallnanotubes (e.g. Carbolex AP grade) which contains a bundled network ofropes, was sonicated at very mild conditions (50 W, 43 KHz) for 15-20minutes in the polymeric solutions (of concentrations between 0.2 wt %to 3 wt %). A black, homogeneous ink-like product was obtained, andmixed with a polystyrene (PS) latex (having a weight averaged molecularweight of 400 kg/mol).

The mixture was then freeze dried (Christ alpha 2-4) overnight and thedry powder was compression molded at 160° C. for 4 minutes at 10 MPa(after 4 circles of degassing).

Cryo-Transmission Electron Microscopy (cryo-TEM) was used to study theproperties of the CNT-latex composition. Cryo-TEM is a particularlysuitable technique for the direct visualization of aggregates ranging insize from about 5-10 nm to 1 micron. The sample is prepared using anewly developed vitrification robot—Vitrobot—in which the relativehumidity is kept close to saturation to prevent water evaporation fromthe sample. A 3 microliter drop of the solution was put on acarbon-coated lacey substrate supported by a TEM 300 mesh copper grid(Ted Pella). After automatic blotting with filter paper, in order tocreate a thin liquid film over the grid, the grid was rapidly plungedinto liquid ethane at its melting temperature, and a vitrified film wasobtained. The vitrified specimen was then transferred under a liquidnitrogen environment to a cryo-holder (model 626, Gatan Inc.,Warrendale, Pa.) into the electron microscope, Tecnai 20—Sphera (FEI),operating at 200 kV with a nominal underfocus of 2-4 micrometer. Theworking temperature was kept below −175° C., and the images wererecorded on a Gatan 794 MultiScan digital camera and processed with aDigital Micrograph 3.6.

Conductivity Measurements

Room temperature electrical conductivity measurements were carried outusing a standard 2 points configuration DC-Conductivity KeithlyElectrometer.

Results and Discussion

Cryo-TEM

The latexes were imaged, and proved to be almost monodisperse, ascomplemented by static light scattering measurements.

The most important parameter to optimize, both for conductivity andmechanical properties of the film, is the strength of the interactionbetween the CNT and the matrix. In the present invention, since theindividual CNT is in contact first with the GA, both properties areenhanced.

Another result is the film homogeneity after compression molding: thedistribution of the CNTs in the film was found to be visuallyhomogeneous, contrary to a film based on a non or less exfoliated bundleof CNTs.

When too much GA is used the film becomes brittle. The same effect isfound when low latex content samples are prepared. However when low GAcontents were used the solubility and exfoliation of the CNT bundles waslimited. Depending on the type of water-soluble first polymer orwater-soluble surfactant, the type of aqueous latex, the ranges at whichthese effects occur differ. The skilled man can determine the effectiveboundaries.

EXAMPLE II Comparative Experiment A

The product of Example I was used for resistivity measurements; theresults were compared with the results given in J. Mater. Sci (supra),in which MWNTs were used. The results of this comparison are given inFIG. 1.

From FIG. 1 it can be seen that the use of SWNTs reduces theconductivity percolation threshold significantly.

EXAMPLES III-V

Example I was repeated, but now using an aqueous dispersion having 1 wt.% SWNT and 1% SDS (resulting in a 1:1 wt. ratio between the SWNT and theSDS). In FIG. 2 the results of the resistivity measurements are given,using polystyrene (PS) as the matrix (Example III); using polymethylmethacrylate (PMMA) as the matrix (Example IV), in comparison with theresults obtained when using GA. In Example V 0.5 wt. % of NaC1 was addedto the GA-dispersed SWNT/latex solution.

EXAMPLES VI-X

In these Examples, the synthesis as well as study of the electricalproperties of composites consisting of individual single-wall,exfoliated carbon nanotubes in a highly viscous polyethylene (PE) matrixis reported. Both nanotubes and PE are dispersed in an aqueous solution.The uniqueness of the method is by employing latex technology for thedispersion of the PE. No nanotube functionalization was necessary, sinceattractive forces between the nanotubes and the polymer chains are notrequired. An environmental advantage of this process is that theSWNT-solution as well as the PE latex are aqueous solutions.

Ethylene emulsion polymerization technology was used to obtain stable PElatices with a solid content of around 2-3 wt % and a particle size ofapproximately 300 nm; stable aqueous SWNT/PE mixtures were achieved,which were suitable for freeze drying. No precipitation of PE latexparticles or nanotubes was observed.

Compressed ethylene was purchased from Air Liquide and used as received.Sodium dodecylsulfate (SDS) was obtained from Merck. CNT-AP grade carbonnanotubes, as in Example I were used. A neutral nickel catalyst was usedto catalyse the emulsion polymerization of ethylene.

The preparation was performed in dried vessels under argon, all solventswere degassed. A previously synthesised neutral nickel (II) catalystcomplex was dissolved in 5 ml of acetone. SDS was dissolved in 95 ml ofwater. Both solutions were added to a mechanically stirredpolymerization reactor, which was subsequently put under ethylenepressure (4 MPa). Polymerization took place during 2 hours. Theresulting latex was poured through a Schleicher & Schuell paper filter(589 black ribbon).

The latex particle size distribution was determined by means of laserdiffraction particle size analysis, using a Beckman Coulter LS 230 smallvolume optical module.

The latex solid content was determined by evaporating the water andacetone, resulting in precipitated polyethylene, which was subsequentlyweighed.

A 1 wt % solution of sodium dodecylsulfate (SDS) in water was prepared,which was subsequently used to make a 1 wt % SWNT solution. Thissolution was sonicated (during 15 minutes at 20 W) and centrifuged at4000 rpm for 20 minutes. The solution is then decanted.

The PE latex was mixed with different amounts of SWNT solution,resulting in composites containing between 0.1 and 1.5 wt % of carbonnanotubes. The mixtures were freeze dried during two days to remove thewater and acetone. Grey composite powders remained, which werecompression moulded at 160° C. and 10 MPa between poly(ethyleneterephthalate) sheets using a Collin Presse 300G. The obtained blackfilms were approximately 0.1 mm thick.

On the films, contact lines were drawn with graphite conductive adhesiveon isopropanol base (Electron Microscopy Sciences), after whichelectrical resistivity was determined using a Keithley 237-6217A set-up.

Cryogenic Transmission Electron Microscopy study was conducted on the PElatex and of a mixture of PE latex with a SWNT solution, as in ExampleI.

Latex particle size and solid content were determined for the differentlattices. Due to the poor solubility of ethylene in water, the polymercontents of the emulsions were low. Table 1 shows the data obtained.

TABLE 1 Latex characteristics Average particle size Solid contentExample [nm] [wt %] VI 300 1.85 VII 360 2.53 VIII 300 2.26 IX 270 1.80 X300 2.15

Cryo-TEM images were made of the PE latex, showing non-spherical PEparticles with diameters up to ˜400 nm, which corresponds with laserdiffraction results. The anisotropicity of the particles, compared withclassical polystyrene emulsion polymerization, indicates that thecrystallization rate is faster than the PE particle growth.

Cryo-TEM images were also obtained of mixtures of PE latex and SWNTsolution. No repulsion between the PE particles and the SWNT wasobserved, indicating that the exfoliated tubes were well dispersedthroughout the PE latex/SWNT solution mixture.

Homogeneous composite films were obtained after freeze-drying andcompression molding. The percolation threshold was lower than 0.5 wt %SWNT in PE.

1. Carbon nanotube reinforced polymer comprising a polymer and carbonnanotubes reinforcing the polymer, the carbon nanotube reinforcedpolymer having a conductivity percolation threshold of less than 10⁶Ω/cm at a carbon nanotube content of 1.5 wt. % and less, wherein thepolymer is selected from a polyamide or a polystyrene based polymer. 2.Carbon nanotube reinforced polymer according to claim 1, wherein thecarbon nanotube content is between 0.1 to 1.5 wt. %.
 3. Carbon nanotubereinforced polymer according to claim 1 having a conductivitypercolation threshold of less than 10⁶ Ω/cm at a carbon nanotube contentof less than 0.5 wt. %.
 4. Carbon nanotube reinforced polymer,comprising a polymer and carbon nanotubes reinforcing the polymer, thecarbon nanotube reinforced polymer having a Relative Size Dimension(RSD) of between 0.85 and 1.0, wherein the RSD is the ratio between theaspect ratio of the carbon nanotubes in the carbon nanotube reinforcedpolymer, and the aspect ratio of the virgin nanotubes, and wherein thecarbon nanotube reinforced polymer product has a conductivitypercolation threshold of less than 10⁶ Ω/cm at a carbon nanotube contentof 1.5 wt. % and less, wherein the polymer is selected from a polyamideor a polystyrene based polymer.
 5. Carbon nanotube reinforced polymeraccording to claim 4, wherein the RSD is at least 0.9.
 6. Carbonnanotube reinforced polymer according to claim 4, wherein the carbonnanotube content is between 0.1 to 1.5 wt. %.
 7. Carbon nanotubereinforced polymer according to claim 4 having a conductivitypercolation threshold of less than 10⁶ Ω/cm at a carbon nanotube contentof less than 0.5 wt. %.